Although the ability of a computer user to access other computers was once considered a luxury or a curiosity, that ability is rapidly becoming a practical necessity. A user typically acquires this ability by coupling a modem to his or her computer's serial I/O port and utilizing the telephone network as the communications medium. The user can then establish communication with remote computers for any desired purpose--paying a bill, obtaining the current price of a share of XYZ Corporation's stock, or doing an on-line patent search.
In the present context, the term "modem" normally refers to an auto-dialing, auto-answering modem, which may be external or internal. An external modem is a stand-alone device having one connector for coupling to the telephone lines and another connector for coupling to the computer's I/O port. The I/O port is commonly an RS-232 interface, which has become a virtual industry standard for asynchronous serial communications in microcomputer devices. The RS-232 interface specifies a set of control signals and voltage levels (nominally .+-.12 volts). An internal modem, as the name implies, is built into the computer enclosure. In either case, the modem and the computer exchange a number of control signals according to a specific protocol so as to condition the modem to accept incoming data calls or place outgoing calls.
Unfortunately, but not surprisingly, the practice sometimes falls short of the theory. A remote computer does not always answer when called; it may be off or it may be busy with other callers. Moreover, the phone lines are at times an imperfect communications medium.
However, in view of the new-found power and flexibility that result from the ability to establish remote communications, computer users have accepted the frustrations of not always being able to establish data communications as a small price to pay.